Ropes are used in numerous applications in the marine and offshore industries as well as in onshore operations. Synthetic Fiber Mooring Rope constructed from high performance polyester is an important emerging technology which promises to advance the economical production of oil and gas from deepwater reservoirs in the Gulf of Mexico (GOM) and elsewhere around the world. The advantages of using synthetic fiber such as polyester for providing station keeping to offshore platforms, for example, Floating Production Systems Offshore (FPSO's) with a taut leg mooring system are huge weight savings, a more efficient system allowing a smaller footprint on the ocean floor, a favorable force vector to restore the platform to its neutral position, reduction in loads on the risers, and the associated lower cost. Synthetic fiber ropes are also commonly used for holding ships in position in port and for loading operations. Ropes carry high tension loads and the service environment is often hostile, potentially causing damage to ropes. Light-weight synthetic fiber mooring ropes including polyester ropes have high strength and adequate stiffness for mooring line applications, but are much more susceptible to damage than their steel counterpart. Close monitoring of their performance is, therefore, a necessary requirement to insure their continued safety and reliability. Inspection schedules, primarily visual, are rigidly enforced to insure their safe operation. In recent years the offshore oil industry has found synthetic fiber ropes to be an important economic enabling technology for the anchoring and position keeping of deepwater offshore floating platforms. A better method of inspection is needed to monitor the strain in these highly loaded ropes as they respond to loads imposed by the platforms offset due to ocean currents and surface wind and the ropes' own weight. The extreme conditions in the marine environment, especially under extreme loading conditions such as hurricanes, and due to possible damage caused by the intrusion of lines and cables from adjacent operations, can reduce the strength of the ropes. Requisite inspection procedures, based primarily on visual techniques for identifying rope damage, are regularly scheduled and rigidly enforced to insure the safe deployment and operation of the ropes. Despite the use of current inspection procedures, however, there have been accidents, damages to property and equipment, injuries and deaths due to unexpected rope failures. The merchant marine industry and the U.S. Navy are particularly concerned about this issue and the offshore oil industry and Department of the Interior's Mineral Management Service have focused numerous studies to evaluate and characterize the problem and potential consequences of rope damage and premature failure.
A reliable in situ method is needed to inspect these primary structure components to ensure their safe reliable performance over an extended period of time. Imposed peak strains are typically large in ropes (several percent) and ultimate strains, depending on the rope architecture, are often from 8 to 20 percent. Real-time knowledge of these strains is highly desirable because such information could be used to develop design guidelines defining “fitness for service” and procedures for retirement and replacement before disastrous consequences occur. The method disclosed herein provides the capability to directly measure the large axial strain imposed on ropes in service using large strain capability plastic optical fibers and a strain measurement method based on “Optical Time Domain Reflectometry” or other methods for measurement of time-of-flight of light. The industry also needs a detection method to discover the presence and severity of local damage, both in the body of the rope and in the proximity of terminations. The method described herein also provides this capability.